1. Field of the Invention
This invention relates to an alkaline-catalyzed process for polymerizing 2-pyrrolidone. In a further aspect, this invention relates to such an alkaline-catalyzed process wherein the reaction product mixture of particulate poly-2-pyrrolidone is treated with acid to reduce the pH of the mixture. In another aspect, this invention relates to a process for the recovery of unreacted 2-pyrrolidone from the afore-described polymerization processes.
2. The Prior Art
Poly-2-pyrrolidone (or nylon 4) is produced by the alkaline-catalyzed polymerization of 2-pyrrolidone.
Polypyrrolidone is generally produced by the batch bulk polymerization. The monomer and catalysts were charged to a reactor and held at polymerization temperature for 20 or more hours to obtain a hard block of polymer. Since, in general, only 30-70% of the monomer was converted to polymer, the product had to be chopped, ground and extracted with water to recover unreacted monomer. The final polymer particle size depended on the extent of grinding, as well as the percent conversion to polymer. Polymerization under agitation, such as in a stirred reactor, produces a product which is a powder or alkaline paste or slurry of particulate polypyrrolidone and polymerizate. At conversions above about 40 percent, the product is powdery. The product is conveniently washed with water to provide a particulate polypyrrolidone without grinding. However, a considerable amount of polymer degradation occurs if the alkaline product is contacted with water while still at the elevated temperature of the polymerization reactor. If, on the other hand, the alkaline product is allowed to stand before contacting with water, it becomes a hard block of polymer requiring the same processing as the batch bulk polymerization product. In our copending U.S. application Ser. No. 792,421, we described an improved continuous process which included a unique acid treatment which eliminated these problems. The present invention is concerned with the broad application of this treatment to substantially eliminate these problems, in both continuous and batch processes.
In polymerizing 2-pyrrolidone using an alkaline catalyst, the recovery of unreacted pyrrolidone has been accomplished by the prior art by several methods all of which begin by washing the incompletely reacted polymerizate with water or a water-miscible solvent to extract the alkaline catalyst and unreacted pyrrolidone. In commonly assigned U.S. application Ser. No. 745,501, filed Nov. 26, 1976, now U.S. Pat. No. 4,050,994 an improved process is described for recovering pyrrolidone from the resulting wash waters. The process, described therein, comprises treating the alkaline wash water with acid followed by a two-stage evaporation. We have found that even though the pre-washed polymerization reaction product slurry is treated with acid in our process, and hence the resulting wash waters may already be neutral or even slightly acidic, the application of acid to such waters facilitates more efficient recovery of 2-pyrrolidone. Various pyrrolidone recovery processes are illustrated by U.S. Pat. Nos. 3,681,l293 and 3,290,329.